1. Field of the Invention
The present invention relates to a switchable wide band receiver front end or front end device for a multiband receiver.
2. Prior Art
Although applicable to any digital, mobile system, the present invention and its underlined problems will be discussed with particular reference to GSM mobile systems.
In general, GSM is designed as an international digital cellular service. Originally, the 900-MHz band was reserved for GSM services, wherein the frequency band from 890 to 915 MHz is reserved for the uplink and the frequency band from 935 to 960 MHz is reserved for the downlink, i.e. sending data from a base station to a mobile station.
Since GSM first entered commercial service in 1992, it has been adapted to work at 1800 MHz for the Personal Communications Networks PCN in Europe and at 1900 MHz for Personal Communications System PCS in the United States. Accordingly, there exist three main GSM systems operating at three different receiving frequency band. Therefore, a mobile station including a wide band receiver front end has to be switchable between the different receiving frequency bands to be operable in different areas having different GSM standards.
FIG. 1 shows a conventional receiver front end for switching between multiple receiving frequency bands RX1 to RXn using a switching means SW which is connected on its input side to an antenna A. The switching means SW of this conventional receiver front end comprises multiple switches S1 to Sn, wherein the number of those switches corresponds to the number of receiving frequency bands RX1 to RXn. Each of the switches S1 to Sn is connected to a narrow band LNA which amplifies the received signal within the predetermined receiving frequency band via a respective bandpass filter BP11 to BP1n. Each of the multiple narrow band low noise amplifiers LNA1 to LNAn is connected on its output side to bandpass filters BP1 to BPn which suppress all signals outside of the corresponding receiving frequency band. Each of the filtered output signals is mixed with a corresponding locally generated mixing signal LO1 to LOn from a frequency synthesizer by multiplication within an associated mixer M1 to Mn and passed to a further narrow-banded bandpass filter BP having a bandpass frequency characteristic to pass only the signal which has a frequency in the range of the chosen physical channel. Accordingly, in a GSM system, the bandwidth of the narrow bandpass filter BP is approximately 200 kHz.
As can be seen from FIG. 1, a conventional receiver front end comprises multiple low noise amplifiers LNA1 to LNAn and multiple mixers M1 to Mn, wherein the number of the low noise amplifiers and the mixers corresponds to the number of different receiving frequency bands which have to be received by the mobile station. For instance, within a mobile station designed to receive a broadcast signal in the downlink frequency bands of GSM 900, GSM 1800 and GSM 1900, three different low noise amplifiers and three different mixers have to be employed. This has the drawback that many components have to be integrated within a mobile station, thus increasing its total production cost and making a further miniaturization difficult.
It is therefore desired to provide a wide band receiver front end for a multiband receiver which is able to switch between multiple receiving frequency bands, but uses a minimum number of different components, thus lowering the production costs and facilitating a miniaturization of the mobile phone.
The invention provides a switchable wide band receiver front end device for a multiband receiver having the features of claim 1.
Particularly, the switchable wide band receiver front end device for a multiband receiver according to the present invention comprises:
a broadcast signal receiving means for receiving broadcast signals within multiple receiving frequency bands, wherein each frequency band is divided into a plurality of channels,
a wide band low noise amplifier connected to said broadcast signal receiving means for amplifying the.broadcast signals of all receiving frequency bands outputting an amplified output signal which is branched to multiple switches of a switching means, wherein the number of said switches corresponds to the number of receiving frequency bands,
multiple filters each connected to one of said switches, wherein each filter has a bandpass filtering characteristic to pass all signals within an associated receiving frequency band,
a mixing means connected to the output side of each filter mixing the filtered signal with a locally generated mixing signal from a frequency synthesizer to produce an intermediate frequency signal, and
a control means outputting a first control signal to said switching means for switching on one of said switches wherein one of said multiple receiving frequency bands is selected and outputting a second control signal to said frequency synthesizer to generate a mixing signal corresponding to the selected receiving frequency band.
In a preferred embodiment of the present invention, a mixing means is an image rejection mixer.
This has the particular advantage that the requirements with respect to the filters are not so high, i.e. the suppression or attenuation of undesired signal components, in particular at the image frequency performed by the filters, does not need to be as high as in the case of using a normal mixing means. Accordingly, a lower number of in-series-connected filters or more simple filters can be used, thus reducing further the production cost and facilitating the miniaturization of the mobile phone.
The principle idea of the present invention resides in using a wide band low noise amplifier which amplifies the broadcast signals of all desired receiving frequency bands and to substitute the plurality of mixers within a conventional receiver front end by only one mixer.
Further preferred embodiments of the invention are listed in the dependent claims.
According to a preferred embodiment, the output side of the mixing means is an IF-filter provided having a filtering characteristic to pass all signals within a narrow frequency band around a predetermined intermediate frequency.
According to a further preferred embodiment, the wide band receiver front end comprises a further switching means having multiple switches each connected to the output side of a filter, wherein said switching means is provided on the input side of said mixing means for inhibiting interactions between the filters.
Providing such a further switching means has the particular advantage that interactions between the different filters which are caused by the direct connection are suppressed.
According to a further preferred embodiment, the mixer is an image reject mixer, wherein on the input side of said image reject mixer, the received signal is branched to a first signal branch and a second signal branch each comprising a normal mixer, wherein the input signal is mixed in the first signal branch with said locally generated mixing signal which is phase-shifted by a first phase-shifting means to produce a first mixing product signal which in turn is phase-shifted by a second phase-shifting means and wherein the input signal is mixed in the second signal branch with said locally generated mixing signal directly to produce a second mixing product signal which is added on the output side of said image rejection mixer to said phase-shifted first mixing product signal to produce said intermediate-frequency signal.
According to a first preferred embodiment, the first and second phase-shifting means of the image reject mixer each performs a phase-shift of 90xc2x0.
According to a further preferred embodiment, the switching means is a high power antenna microwave monolithic integrated circuit having multiple receiving faults and a switching speed of about 100 ns.
According to a further preferred embodiment, the receiving frequency bands are downlink frequency bands of a GSM telecommunication system.
According to a further preferred embodiment, the wide band receiver front end is capable of receiving three different frequency bands, wherein the first receiving frequency band is the downlink frequency band of GSM 900, the second receiving frequency band is the downlink frequency band of GSM 1800 and the third receiving frequency band is the downlink frequency band of GSM 1900.
According to a further preferred embodiment, the IF-filter has a narrow frequency passband of 200 KHz corresponding to a frequency width of a physical GSM channel.
According to a further preferred embodiment, the control means detects automatically on which receiving frequency band data is broadcast from a transmitting base station and generates said first control signal such that this receiving frequency band is selected.
According to a further preferred embodiment, the filters are selected from a group consisting of surface acoustic wave filters and dielectric filters. In principle, all kinds of filters are possible, such as helical filters, multilayer filters, etc.
According to a further preferred embodiment, said wide band low noise amplifier amplifies the received broadcast signal within a wide frequency band which includes more than one of the desired multiple receiving frequency bands.
According to a further preferred embodiment, said wide frequency band of said wide band low noise amplifier includes the downlink frequency bands of. GSM 900, GSM 1800 and GSM 1900.
According to a further preferred embodiment, said wide band frequency band ranges from 100 MHz to 2000 MHz.
According to a further preferred embodiment, the locally generated mixing signals are chosen such that the frequency difference between said mixing signals is minimal.
According to a further preferred embodiment, the frequency of the intermediate frequency signal which is output by said mixing means is 400 MHz.
According to a further preferred embodiment, the signal noise components at the image frequency are suppressed.
According to a further preferred embodiment, the low noise amplifier only covers two or more frequency bands.
According to a further preferred embodiment, the switches and the filters are placed between the antenna and the low noise amplifier.
According to a further preferred embodiment, at least some of the switches are replaced by matching circuits.
According to a further preferred embodiment, more narrow banded filters are provided for filtering one or more of the frequency bands, instead of one filter covering the entire receiving band, preferably the GSM 1800 is split into two filters, the first covering the band from 1805 to 1845 MHz and the second covering the band from 1845 to 1880 MHz.